This invention relates to a self locking drive system for rotating the plugs on the head of a nuclear reactor which is able to restrain plug motion if a seismic event should occur during reactor refueling.
Due to the highly radioactive nature of the fuels used in nuclear reactors, it is essential to conduct all fuel handling operations to preclude accidents which would release radioactive materials into the atmosphere. One natural force which must be seriously considered when designing a reactor is the possibility of an earthquake.
In several Liquid Metal Fast Breeder Reactors such as Clinch River Breeder Reactor Plant (CRBRP), large eccentric rotating plugs are employed to permit refueling through holes in the head of the reactor core. Since uncontrolled rotation of the plugs during refueling could rupture fuel assemblies which contain radioactive fuels and render the reactor inoperative for an indefinite period of time, a self locking device is necessary to prevent unwanted plug rotation. It is noted that massive eccentric components mounted on the plugs create large torques when subject to possible horizontal seismic accelerations. However, drive units that are well known in the art which rotate each plug via a pinion and bull gear are unable to restrain the plug without gear breakage since the seismic torque is an order of magnitude higher than normal drive torque. Since counterbalancing the eccentric load is impractical, some other method to prevent seismic rotation is necessary.
Several possible designs have been formulated for locking the plugs during refueling. One such design which was the original approved design for the CRBRP utilized a "synchronized worm". In this design a D.C. servomotor drives a drive train which is drivingly connected to the plug via a bull gear. The bull gear is integrally attached atop the plug via a riser assembly. Also connected to the drive train is another gear train synchronizing a large worm gear with the bull gear. The purpose of this "synchronized worm" is to prevent rotation of the bull gear due to seismic torque. Normally the worm doesn't touch the bull gear. When a seismic event occurs a shear coupling which is included in the normal drive train breaks. This disconnects the normal drive train allowing it to freewheel without damaging the gears, and causes the worm to engage the bull gear thus taking the high seismic torque and locking the bull gear.
However, use of this design creates many problems. The bull gear must have helical rather than spur type teeth. Not only would it be extremely difficult to manufacture a worm gear which is large enough to engage the bull gear but the worm to helix conformity would be poor. Further, it would be difficult to properly synchronize the drive train which is connected to the bull gear and the drive train which is connected to the worm gear. For these reasons this original design for use on the CRBRP was abandoned.
It is an object of the present invention to provide a self locking drive system for rotating the plugs on the head of a nuclear reactor.
It is a further object of this invention to provide a drive system which is able to withstand torque resulting from a seismic occurrence.